Human infertility affects around 15% of couples, and the male contribution to it is found in near 50% of all cases [Pilch, B.; Mann, M., 2006; Robert, M.; Gagnon, C., 1994]. One of the medical conditions of male infertility is azoospermia, which is characterized by a non-measurable level of sperm in the semen [Edwards, J. J. et al, 1981]. Azoospermia is diagnosed in 20% of non-fertile men and has two forms: obstructive azoospermia (OA) and non-obstructive azoospermia (NOA). OA is caused by a physical obstruction in the male reproductive tract. The biological outcome of OA is thus identical to that of vasectomy, which is a surgical severance of vas deference. NOA is a more complicated infertility syndrome that can be classified as maturation arrest, Sertoli-cell-only syndrome and hypospermatogenesis [Martinez-Heredia, J. et al, 2008]. The only currently used method to definitively distinguish between OA and NOA syndromes is a testicular biopsy, which is an invasive surgical procedure [Fung, K. Y., 2004; Yamakawa, K. et al, 2007]. Thus, there is an urgent need for alternative non-invasive approaches with better diagnostic potential. Differential diagnosis of normal, NOA, and OA (or post-vasectomy, PV) conditions is required for the following reasons: (i) in infertile patients, use of molecular markers capable of differentiating NOA and OA may eliminate the need for a diagnostic testicular biopsy; (ii) in healthy individuals that underwent vasectomy, markers capable of differentiating normal and PV seminal plasma will reveal if vasectomy was successful.
Proteins are considered as promising biomarkers for clinical diagnostics. However, no biomarkers currently exist to definitively diagnose OA or NOA. Blood plasma levels of inhibin B or FSH were proposed for NOA diagnosis, but these molecules had poor specificity and sensitivity [Wang, J. et al, 2009; Lilja, H.; et al, 1987]. Protein levels in local fluids, such as seminal plasma, may have a better potential for diagnostics [Searle, B. C. et al, 2008]. For example, PTGDS protein has been recently proposed as a seminal plasma biomarker for diagnosis of OA [Elias, J. E. and Gygi, S. P., 2007]. However, PTGDS could not distinguish NOA from normal and OA (PV) groups with high confidence.
Recent progress in biological mass spectrometry facilitated identification of several thousand proteins in biological fluids [Choi, H. and Nesvizhskii, A., 20; Sardana, G. et al, 2008]. While identification of proteins is successfully used in biomedical research, routine quantification of proteins by mass spectrometry is still a challenge that requires considerable methodological and instrumental advances. Quantitative selected reaction monitoring (SRM) assays [Poliakov, A. et al, 2009; Thimon, V. et al, 2008] were introduced as mass spectrometry means to compete with antibody-based ELISAs that were widely used for verification of biomarkers in large numbers of clinical samples. Verification of biomarkers by SRM assays is an emerging field of proteomics with just several studies to date [Sullivan, R. et al, 2007; Virji, N. and Naz, R. K., 1995; Odet, F. et al, 2008]. The proteome of seminal plasma is as complex as the proteome of blood serum and contains large amounts of semenogelin, kallikrein 3, and other high-abundance proteins [Sawane, M. V. et al, 2002; Takayama, T. et al, 2005a, Takayama, T. et al, 2005b].